Numerous medications are administered in the form of aerosols. In the past, compressed-gas aerosols have mainly been used for this purpose. Owing to the known problems concerning environmentally harmful propellants, therapy with powder aerosols gained increasingly in importance.
In this type of drug, the active ingredient is used in a suitable formulation, e.g. in a pure form as soft pellets or as a mixture with suitable auxiliaries, e.g. lactose-monohydrate.
When inhaled by the patient, the formulation is dispersed into the stream of breathing air, only sufficiently fine particles reaching the site of action, the lung, while coarser agglomerates and particles are precipitated in the upper respiratory tracts and in the throat area.
An important objective of the development of powder inhalers and formulations is therefore to maximize the proportion of fine powder and to minimize the proportion of coarse powder.
The energy required for the dispersion of the formulation in powder inhalers is usually obtained from the stream of breathing air of the patient. According to experience, this energy, which is available only to a limited extent, is not sufficient for a quantitative dispersion into respirable particles of active ingredient.
Moreover, numerous aerosol formulations (e.g. adhesive powder mixtures) contain auxiliaries (e.g. lactose monohydrate) which, based on their grain size, are mainly precipitated as coarse powder in the upper respiratory tracts. The finer particles of active ingredient adhering to these auxiliaries are thus likewise precipitated in the upper respiratory tracts.
This is particulary disadvantageous when the active ingredient--such as, for example, in the case of corticoids--shows an undesired local effect. In the past, for instance, local fungal infections were often described as a consequence of aerosol therapy with corticoids.
In the case of compressed-gas aerosols, numerous precipitation systems, called "spacers", were described to reduce this precipitation of active ingredient in the throat area.
These precipitation systems contain a number of advantages:
Firstly, the speed of the spray jet emerging from the spray heads is reduced, as a result of which the precipitation due to impaction in the throat area is reduced.
Moreover, in particular in the case of large-volume spacers, a quantitative evaporation of the propellant can be achieved, as a result of which the droplet size of the aerosol droplets is reduced. Finally, there is the possibility, in the case of large-volume spacers, to dissociate the inhalation of the patient from the release of the active ingredient and thus to reduce the coordination problems of numerous patients.
While spacers of this type are generally in use for compressed-gas aerosols, they have not previously been used in powder aerosols. This is surprising in so far as the principle problems of the precipitation of appreciable quantities of active ingredient in the throat likewise exist in powder aerosols.
Precipitation systems which are to be used in conjunction with powder aerosols however, must have different design properties from precipitation systems such as are in use for compressed-gas aerosols.
The invention is based on the object of developing a precipitation system which is suitable for combination with powder inhalers and leads to an appreciable reduction in the depositing of non-respirable active ingredient and auxiliary in the throat area. On the other hand, the desired, respirable quantity of active ingredient is not to be adversely affected by the precipitation system.
According to the invention, this object is achieved in that the precipitation system comprises a collection tube which can be fitted onto the powder inhaler with a mouthpiece and a centrifugal precipitator which is arranged in the collection tube and has at least one spin-producing surface which produces a spin flow in the collection tube, as a result of which the heavy coarse powder particles are precipitated on the inner wall of the collection tube while the lighter fine powder particles pass into the mouthpiece due to a flow which is essentially restricted to the area surrounding the tube axis.
The flow resistance of the centrifugal precipitator advantageously lies between 0.3 mbar and 2 mbar for an inhalation flow of 60 l/min.
The collection tube advantageously has a volume between 10 cm.sup.3 and 300 cm.sup.3, preferably between 20 cm.sup.3 and 100 cm.sup.3.
The spin-producing surface can consist of a flat or curved vane surface mounted in the collection tube. However, the spin-producing surfaces preferably consist of elliptical segments which fill the entire cross-section of the collection tube, are inclined at an acute angle .alpha. to the tube axis, and whose periphery terminates flush with the inner wall of the collection tube.
Particularly good precipitation results are achieved if the spin-producing segment surfaces are designed such that a central opening remains in the tube centre at the inner edges of the segments.
The angle .alpha. between the tube axis and the segment surfaces preferably lies within the range from 400 to 700. The optimum ratio of the inside diameter of the collection tube to the diameter of the central opening lies within the range from 2 to 10.
The system described here is distinguished particularly by the following properties:
In comparison with precipitation systems according to the principle of impact precipitation, the system described here shows no appreciable inherent pressure loss in use. This prevents the volume flow in the inhaler being reduced during the application, which would have a disadvantageous effect on the dispersion of the formulation and the metering accuracy. Moreover, the flow resistance of the entire system, which is unpleasant for the patient, is not noticeably increased.
A further characteristic of the system presented is that it is distinguished by a small volume, as a result of which it differs, in particular, from spacers, such as are customary in compressed-gas aerosols. It is thus prevented that an appreciable part of the air stream inhaled by the patient is not loaded with active ingredient. Moreover, the ensuing small construction of the appliance allows it to be carried with the patient without effort.
A further advantage of the system presented here consists in the fact that it can be dismantled and cleaned in a simple manner by the patient.